1. Field of the Invention
This following invention relates to RF power amplifiers such as those used for wireless applications.
2. Description of the Related Art
Radio frequency (RF) power amplifiers (PAs) are commonly used within the transmit path for wireless applications to boost the transmitted power provided to the antenna. They are typically designed to provide power into a fixed impedance load, such as 50 ohm, and are designed to operate from a fixed supply voltage, such as 3.5V. At its output, the power amplifier (PA) has an output network that is formed from passive components, such as inductors, capacitors, and transformers that are arranged in a tuned configuration. The networks formed from these passive elements are resonant at frequencies near that of the desired radio frequency (RF) carrier. Within this narrow frequency range, the circuit provides impedance transformation. Without impedance transformation, the maximum power that could be delivered by the power amplifier to a fixed load would be limited by the amplifier's supply voltage. With impedance transformation, the power amplifier can provide an output power that is higher than this limit.
Three important performance metrics for RF power amplifiers are output power, efficiency, and linearity. It is important that the PA meets a desired transmit power level. This assures a good connection between the transmitter and receiver and for many systems is required for type approval testing. It is also important that the PA transmits with high efficiency. This minimizes battery drain and heat generation. Finally, it is important that the PA transmits with good linearity to assure that it does not generate power at frequencies other than those that are desired.
FIG. 1 shows an example of an RF power amplifier and associated output network that are well known in the field. The amplifier's final stage is formed from transistor 102 and inductor 103. Capacitor 109 acts as a DC blocking element and elements 105, 106, and 107 perform impedance transformation, transforming the load impedance 111 from its actual value, RL, to new impedance, ZT, seen by the final PA stage. The ratio RL/ZT is referred to as the transformation ratio. The load impedance RL is usually purely real. Components in the amplifier's output network are usually chosen so that ZT is strictly real as well.
FIG. 2 shows an example of a common transmit chain for wireless transmitters. The PA's amplification stages, 202, are followed by an output network, 204, which is followed by an isolator, 206, and then a directional coupler, 208. An isolator is a passive device that provides the PA with a more controlled load impedance despite large variations in antenna impedance. The directional coupler generates an output signal, 210, that represents the level of power being transmitted to the antenna. This measure is fed back to the amplifier stages where it is used to control power levels being generated. In some cases, the purpose of this loop is to reduce variations in transmitted power from device to device and over conditions such as temperature. In other cases, this loop is used to assure that the maximum power provided to the antenna remains below a maximum limit as set by safety considerations.
Thus, power amplifiers are an important component of wireless technology and improvements in operation of the power amplifier are desirable.